Uv curable inkjet inks

ABSTRACT

A liquid UV curable inkjet ink includes one or more photoinitiators, an organic colour pigment, and a polymerizable composition containing at least one monofunctional polymerizable compound and at least one polyfunctional polymerizable compound, wherein the organic colour pigment is present in an amount of 6.0 to 13.0 wt % based on the total weight of the liquid UV curable inkjet ink; the at least one polyfunctional polymerizable compound is present in an amount of at least 20.0 wt % based on the total weight of the polymerizable composition; and the UV curable inkjet ink has a viscosity at 45° C. and a shear rate of 10 s−1 of at least 16.0 mPa·s.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a 371 National Stage Application ofPCT/EP2017/056541, filed Mar. 20, 2017. This application claims thebenefit of European Application No. 16161328.6, filed Mar. 21, 2016,which is incorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to UV curable inkjet inks for multi-colourinkjet printing methods.

2. Description of the Related Art

Industrial inkjet printing systems are increasingly replacing analogueprinting systems, like offset and flexography, because of theirflexibility in use and variable data printing capability. UV curableinkjet inks are particularly preferred because high quality colourimages can be printed on non-absorbing ink-receivers, such as plastic ormetal. For many applications, colour images printed on suchnon-absorbing ink-receivers should exhibit a high scratch resistance.Usually this is obtained by applying thick, highly cross-linked inklayers. Mostly this functions well for metal ink-receivers; however forplastic substrates in addition to scratch resistance often also asufficient flexibility is required.

One approach is to find an optimal balance between monofunctional andpolyfunctional polymerizable compounds, as exemplified by EP 2399965 A(AGFA)). Monofunctional monomers generally increase the flexibility,while polyfunctional monomers increase scratch resistance. However,scratch resistance and flexibility of an ink layer stand in a directrelation as shown by FIG. 1, where an increase in scratch resistance Sresults in a decrease of flexibility F and vice versa. For someapplications, the attainable compromise in scratch resistance andflexibility remains insufficient.

A second approach is to replace part of the monomers in the inkjet inkby one or more reactive specific prepolymers or polymers. For example,US 2004024078 (SEIREN) discloses UV curable inkjet inks including acolouring component, a reactive oligomer and/or a reactive prepolymer, areactive diluent and a photoinitiator, wherein a polymer of the reactiveoligomer and/or reactive prepolymer and a polymer of the reactivediluent have a glass transition point of 0° C. to 70° C. The cured filmof such an ink exhibited good flexibility, scratch resistance andadhesion. The ink compositions have a rather high viscosity of 60 to 800mPa·s at 25° C., thus requiring high jetting temperatures of 60° ormore.

The viscosity of UV curable colour inkjet inks is controlled byselecting appropriate low-viscosity monomers and oligomers. As viscositytends to increase somewhat with the pigment concentration, UV curablecolour inkjet inks in the market generally contain 1 to 5 wt % of anorganic colour pigment based on the total weight of the inkjet ink. UVcurable white inkjet inks generally contain a much higher concentrationof an inorganic pigment, usually at least 15 wt % of titanium dioxide,in order to obtain a very opaque white layer. A slightly higherviscosity is tolerated for a white inkjet ink as it slows down thesedimentation of the white pigment which has a larger density and isalso present in the inkjet ink at larger average particle sizes forobtaining good opaqueness.

An alternative inkjet technology employs so-called hot melt or phasechange inkjet inks that are solid at room temperature and after meltingare jetted at high jetting temperatures of 80° C. to 140° C. forobtaining a jetting viscosity of about 13 mPa·s. Such a UV curable phasechange inkjet ink is disclosed e.g. by EP 1456307 A (3D SYSTEMS)). Adisadvantage of jetting at very high temperatures is that the number ofsuitable ink-receivers becomes limited to non-thermosensitive substratesand that the energy consumption of the inkjet printing device increasesdrastically for melting the inkjet ink.

There still remains a need for UV curable inkjet inks exhibiting goodflexibility and scratch resistance, while maintaining high cure speedand reliability necessary for an economical industrial inkjet printingprocess.

SUMMARY OF THE INVENTION

In order to overcome the problems described above, preferred embodimentsof the present invention have been realized with liquid UV curableinkjet inks as defined below.

Preferred embodiments of the invention have also been realised with a UVcurable inkjet printing method as defined below.

It was surprisingly found that a better compromise in scratch resistanceand flexibility was obtained by increasing the viscosity at jetting to16.0 mPa·s or more, increasing the organic pigment concentration between6.5 wt % and 13.0 wt % in the UV curable inkjet ink, and controlling theamount of polyfunctional polymerizable compounds to a certain range.These measures lead to thinner ink layers having also an improved touchand feel on e.g. plastics and textiles.

The person skilled in the art of inkjet printing has always avoidedprinting at higher viscosities, since printing reliability is reduced bythe increasing number of satellites (see FIGS. 3a and 3b ). However, ifboth jetting viscosity and pigment concentration are controlled tovalues as prescribed in our invention, the number of satellitessurprisingly even decreases with increasing jetting viscosity (see FIG.4).

Further advantages and preferred embodiments of the present inventionwill become apparent from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the relation between the scratch resistance S and theflexibility F of a UV curable inkjet ink layer.

FIG. 2 is a schematic representation of a print head 1 ejecting an inkdroplet 2 onto an ink-receiver 3. The ejected ink droplet 2 forms a tail4 having a length L before splitting up in a main ink droplet 5 and oneor more satellites. A satellite is an unwanted ink droplet producedbehind the main ink droplet, which either merges with the main inkdroplet (fast satellite 6) or drifts away from the main drop (slowsatellite 7). On landing on the ink-receiver 3, the main ink droplet 5forms an ink dot 8, while the slow satellite 7 forms a smaller secondaryink dot 9.

FIG. 3.a shows the general relation between the tail length L of anejected droplet and the jetting viscosity V, while FIG. 3.b shows thenumber of satellites #S as a function of the tail length L.

FIG. 4 is a graph of the results of Example 2 depicting the number ofsatellites #S as a function of viscosity and inkjet inks havingdifferent pigment concentration.

FIG. 5 is a photograph of an apparatus for determining flexibility.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Definitions

The term “alkyl” means all variants possible for each number of carbonatoms in the alkyl group i.e. methyl, ethyl, for three carbon atoms:n-propyl and isopropyl; for four carbon atoms: n-butyl, isobutyl andtertiary-butyl; for five carbon atoms: n-pentyl, 1,1-dimethyl-propyl,2,2-dimethylpropyl and 2-methyl-butyl, etc.

Unless otherwise specified a substituted or unsubstituted alkyl group ispreferably a C₁ to C₆-alkyl group.

Unless otherwise specified a substituted or unsubstituted alkenyl groupis preferably a C₁ to C₆-alkenyl group.

Unless otherwise specified a substituted or unsubstituted alkynyl groupis preferably a C₁ to C₆-alkynyl group.

Unless otherwise specified a substituted or unsubstituted aralkyl groupis preferably a phenyl or naphthyl group including one, two, three ormore C₁ to C₆-alkyl groups.

Unless otherwise specified a substituted or unsubstituted alkaryl groupis preferably a C₇ to C₂O-alkyl group including a phenyl group ornaphthyl group.

Unless otherwise specified a substituted or unsubstituted aryl group ispreferably a phenyl group or naphthyl group

Unless otherwise specified a substituted or unsubstituted heteroarylgroup is preferably a five- or six-membered ring substituted by one, twoor three oxygen atoms, nitrogen atoms, sulphur atoms, selenium atoms orcombinations thereof.

The term “substituted”, in e.g. substituted alkyl group means that thealkyl group may be substituted by other atoms than the atoms normallypresent in such a group, i.e. carbon and hydrogen. For example, asubstituted alkyl group may include a halogen atom or a thiol group. Anunsubstituted alkyl group contains only carbon and hydrogen atoms

Unless otherwise specified a substituted alkyl group, a substitutedalkenyl group, a substituted alkynyl group, a substituted aralkyl group,a substituted alkaryl group, a substituted aryl and a substitutedheteroaryl group are preferably substituted by one or more constituentsselected from the group consisting of methyl, ethyl, n-propyl,isopropyl, n-butyl, isobutyl and tertiary-butyl, ester, amide, ether,thioether, ketone, aldehyde, sulfoxide, sulfone, sulfonate ester,sulphonamide, —Cl, —Br, —I, —OH, —SH, —CN and —NO₂.

Inkjet Inks

A liquid UV curable inkjet ink according to a preferred embodiment ofthe present invention contains one or more photoinitiators, an organiccolour pigment and a polymerizable composition containing at least onemonofunctional polymerizable compound and at least one polyfunctionalpolymerizable compound, wherein the organic colour pigment is present inan amount of 6.0 to 13.0 wt % based on the total weight of the liquid UVcurable inkjet ink; the at least one polyfunctional polymerizablecompound is present in an amount of at least 20.0 wt % based on thetotal weight of the polymerizable composition; and the UV curable inkjetink has a viscosity at 45° C. and a shear rate of 10 s⁻¹ of at least16.0 mPa·s.

The term “liquid” in liquid UV curable inkjet ink means that inkjet inkis a liquid at room temperature (25° C.), thereby stating that theliquid UV curable inkjet ink is not a so-called UV curable phase changeor hot melt inkjet ink.

The organic colour pigment is preferably dispersed in the liquid vehicleof the inkjet ink by a polymeric dispersant. The liquid UV curableinkjet ink may contain a dispersion synergist to improve the dispersionquality and stability of the ink. Preferably, at least the magenta inkcontains a dispersion synergist. A mixture of dispersion synergists maybe used to further improve dispersion stability.

The liquid UV curable inkjet ink is preferably a so-called 100% solidsUV curable inkjet ink. This means that no solvents, i.e. water ororganic solvents, are present. However sometimes a small amount,generally less than 1 or 2 wt % of water based on the total weight ofthe inkjet ink, can be present. This water was not intentionally addedbut came into the inkjet ink via other components as a contamination,such as for example hydrophilic monomer.

The liquid UV curable inkjet ink preferably does not contain an organicsolvent. But sometimes it can be advantageous to incorporate a smallamount of an organic solvent to improve adhesion to the surface of asubstrate after UV-curing. In this case, the added solvent can be anyamount in the range that does not cause problems of solvent resistanceand VOC. The liquid UV curable inkjet ink preferably contains 0 to 10 wt%, more preferably no more than 5.0 wt % of an organic solvent based onthe total weight of the UV curable inkjet ink.

The liquid UV curable inkjet ink contains one or more photoinitiatorsfor the polymerization of the polymerizable composition after UVexposure. In a preferred embodiment of the liquid UV curable inkjet ink,at least one of the one or more photoinitiators is selected from thegroup consisting of a polymeric photoinitiator, a polymerizablephotoinitiator and a photoinitiator containing a plurality ofphotoinitiating groups. Preferably the liquid UV curable inkjet ink iscured by UV LEds having an emission wavelength higher than 360 nm. Forthis reason, the one or more photoinitiators preferably include anacylphosphine oxide photoinitiator and a thioxanthone photoinitiator.

The polymerizable composition contains at least one monofunctionalpolymerizable compound and at least one polyfunctional polymerizablecompound. If a polymerizable photoinitiator is present in the inkjetink, then this photoinitiator is also regarded to be part of thepolymerizable composition. The at least one polyfunctional polymerizablecompound is present in at least 20 wt % based on the total weight of thepolymerizable composition.

In a more preferred embodiment of the liquid UV curable inkjet ink, theat least one polyfunctional polymerizable compound is present in anamount of 25.0 to 50.0 wt % based on the total weight of thepolymerizable composition. In the latter case, an optimal compromisebetween scratch resistance and flexibility is obtained.

In a preferred embodiment of the liquid UV curable inkjet ink, thepolymerizable composition is present in amount smaller than 75.0 wt %,more preferably smaller than 70.0 wt % and most preferably smaller than68.0 wt % based on the total weight of the liquid UV curable inkjet ink.

The UV curable inkjet ink has a viscosity at 45° C. and a shear rate of10 s⁻¹ of at least 16.0 mPa·s, preferably at least 20.0 mPa·s., morepreferably 25.0 to 65.0 mPa·s.

The UV curable inkjet ink has a viscosity of at least 16.0 mPa·s at ashear rate of 10 s⁻¹ and a temperature of 45° C., more preferably atemperature of 50° C.

The liquid UV curable inkjet ink has at room temperature (25° C.)preferably a viscosity of at least 40.0 mPa·s, more preferably at least50.0 mPa·s and most preferably between 60 mPa·s and 250 mPa·s. Above 250mPa·s the pumping around of the UV curable inkjet ink requires morepowerful pumps, which represents a financial penalty for themanufacturing of the inkjet printer. Above 40.0 mPa·s to 50.0 mPa·s, anenhanced dispersion stability of the organic colour pigment in theliquid UV curable inkjet ink is observed.

The surface tension of the liquid UV curable inkjet ink is preferably inthe range of 20 mN/m to 30 mN/m at 25° C., more preferably in the rangeof about 22 mN/m to about 25 mN/m at 25° C.

The liquid UV curable inkjet ink may further also contain at least oneinhibitor or stabilizer for improving the thermal stability of the ink.

The liquid UV curable inkjet ink may further also contain at least onesurfactant for obtaining good spreading characteristics on a substrate.

For printing multi-colour images, the liquid UV curable inkjet ink ispart of a UV curable inkjet ink set. A preferred UV curable inkjet inkset for printing different colours contains at least one liquid UVcurable inkjet ink according to the invention. A simultaneousimprovement for scratch resistance and flexibility is already observedwith a single liquid UV curable colour inkjet ink according to theinvention, but preferably a plurality liquid UV curable inkjet ink arepresent having a composition according to the invention

In a particularly preferred embodiment, the UV curable inkjet ink setcontains at least three liquid UV curable colour inkjet inks eachcontaining one or more different organic colour pigments present in anamount of at least 5.0 wt %, more preferably at least 6.0 wt % and mostpreferably at least 6.5 wt % based on the total weight of the liquid UVcurable colour inkjet ink; with the proviso that at least liquid UVcurable inkjet ink is present having at least 6.0 wt % and morepreferably at least 6.5 wt % based on the total weight of the liquid UVcurable colour inkjet ink.

The UV curable inkjet ink set is preferably a UV curable CMYK or CRYKinkjet ink set. This UV curable inkjet ink set may also be extended withextra inks such as red, green, blue, and/or orange to further enlargethe colour gamut of the image. The UV curable inkjet ink set may also beextended by the combination of full density inkjet inks with lightdensity inkjet inks. The combination of dark and light colour inksand/or black and grey inks improves the image quality by a loweredgraininess.

The curable inkjet ink set may also include a colourless UV curableinkjet ink, such as a varnish or a primer. A varnish is used to enhancethe glossiness of inkjet printed colour images. A primer can be used toimprove the adhesion on difficult substrates like glass andpolypropylene.

The curable inkjet ink set preferably also includes liquid UV curablewhite inkjet ink. The liquid UV curable white inkjet ink preferablycontains a titanium dioxide pigment, preferably a rutile pigment, havingan average particle size larger than 180 nm.

White inkjet inks are generally used for so-called “surface printing” or“backing printing” to form a reflection image on a transparentsubstrate. In surface printing, a white background is formed on atransparent substrate using a white ink and further thereon, a colourimage is printed, where after the formed final image is viewed from theprinted face. In so-called backing printing, a colour image is printedon a transparent substrate using colour inks and then a white ink isapplied onto the colour inks, and the colour image is observed throughthe transparent substrate. In a preferred embodiment the liquid UVcurable colour inkjet ink is jetted on at least partially cured whiteinkjet ink. If the white ink is only partially cured, an improvedwettability of the colour inkjet ink on the white ink layer is observed.

The UV curable inkjet ink set is preferably a free radical curableinkjet ink set. It was found that cationically curable inkjet inks posedproblems of jetting reliability due to UV stray light. UV stray lighthitting the nozzle plate of an inkjet print head results into failingnozzles due to clogging by cured ink in the nozzle. Unlike free radicalcurable ink where radical species have a much shorter lifetime, acationic curable ink continues to cure once an acid species has beengenerated by UV light in the nozzle.

Organic Colour Pigments

The liquid UV curable inkjet ink contains an organic colour pigment inan amount of 6.0 to 13.0 wt % based on the total weight of the liquid UVcurable inkjet ink. An organic colour pigment includes in its chemicalmolecular structure carbon atoms, hydrogen atoms and at least one ofsulphur atoms, oxygen atoms, nitrogen atoms, and selenium atoms. Carbonblack and metal oxides, such as cobalt oxide or titanium dioxide areconsidered inorganic pigments. Organic colour pigments may howeverinclude a metal atom or ion, such as e.g. copper phthalocyaninepigments.

The organic colour pigment is preferably selected of the groupconsisting of may a cyan, magenta, yellow, red, orange, violet, blue,green, and brown, organic colour pigment. A colour pigment may be chosenfrom those disclosed by HERBST, Willy, et al. Industrial OrganicPigments, Production, Properties, Applications. 3rd edition. Wiley-VCH,2004. ISBN 3527305769.

Suitable organic colour pigments are disclosed in paragraphs [0128] to[0138] of WO 2008/074548 (AGFA GRAPHICS).

In a preferred embodiment, the liquid UV curable inkjet ink includes anorganic colour pigment having the numbers below described in the ColourIndex.

In a preferred embodiment, the liquid UV curable inkjet ink is a liquidUV curable yellow inkjet ink including an organic colour pigmentselected from the group consisting of C.I Pigment Yellow 120, C.IPigment Yellow 150, C.I Pigment Yellow 151, C.I Pigment Yellow 155 andC.I Pigment Yellow 180, more preferably selected from the groupconsisting of C.I Pigment Yellow 151 and C.I Pigment Yellow 155.

A preferred organic colour pigment for the liquid UV curable cyan inkjetink is C.I. Pigment Blue 15:4.

A preferred organic colour pigment for the liquid UV curable magenta orred inkjet ink is a quinacridone pigment, a diketopyrrolopyrrole pigmentor mixed crystals thereof.

Mixed crystals are also referred to as solid solutions. For example,under certain conditions different quinacridones mix with each other toform solid solutions, which are quite different from both physicalmixtures of the compounds and from the compounds themselves. In a solidsolution, the molecules of the components enter into the same crystallattice, usually, but not always, that of one of the components. Thex-ray diffraction pattern of the resulting crystalline solid ischaracteristic of that solid and can be clearly differentiated from thepattern of a physical mixture of the same components in the sameproportion. In such physical mixtures, the x-ray pattern of each of thecomponents can be distinguished, and the disappearance of many of theselines is one of the criteria of the formation of solid solutions. Acommercially available example is Cinquasia™ Magenta RT-355-D from CibaSpecialty Chemicals.

Also mixtures of pigments may also be used in the liquid UV curableinkjet ink.

Pigment particles in inkjet inks should be sufficiently small to permitfree flow of the ink through the inkjet-printing device, especially atthe ejecting nozzles. It is also desirable to use small particles formaximum colour strength and to slow down sedimentation.

The numeric average pigment particle size is preferably between 0.050and 1 μm, more preferably between 0.070 and 0.300 μm and particularlypreferably between 0.080 and 0.200 μm. Most preferably, the numericaverage pigment particle size is no larger than 0.150 μm. An averageparticle size smaller than 0.050 μm is less desirable for decreasedlight-fastness. The determination of the numeric average particlediameter is best performed by photon correlation spectroscopy at awavelength of 633 nm with a 4 mW HeNe laser on a diluted sample of thepigmented inkjet ink. A suitable particle size analyzer used was aMalvern™ nano-S available from Goffin-Meyvis. A sample can, for example,be prepared by addition of one drop of ink to a cuvette containing 1.5mL ethyl acetate and mixed until a homogenous sample was obtained. Themeasured particle size is the average value of 3 consecutivemeasurements consisting of 6 runs of 20 seconds.

The organic colour pigment in preferably present in an amount of 6.5 wt% to 13.0 wt %, more preferably 7.0 wt % to 12.0 wt %, and mostpreferably 8.0 wt % to 11.0 wt %, with the weight percentage (wt %)based on the total weight of the liquid UV curable inkjet ink.

Inorganic Colour Pigments

The UV curable inkjet ink set for printing different colours containinga plurality of liquid UV curable inkjet inks includes preferably atleast one black or grey UV curable inkjet ink. The liquid UV curableblack inkjet ink includes preferably a carbon black pigment, morepreferably further also a β-copper phthalocyanine pigment having anaverage particle size smaller than 200 nm. By including a β-copperphthalocyanine pigment in the black inkjet ink, images can be printedhaving an appealing neutral black or grey colour instead of a brownishblack or grey colour.

The UV curable inkjet ink set for printing different colours containinga plurality of liquid UV curable inkjet inks preferably includes also aliquid UV curable white inkjet ink.

Suitable white pigments are given by Table 2 in [0116] of WO 2008/074548(AGFA GRAPHICS). The white pigment is preferably a pigment with arefractive index greater than 1.60. The white pigments may be employedsingly or in combination. Preferably titanium dioxide is used as pigmentwith a refractive index greater than 1.60. Suitable titanium dioxidepigments are those disclosed in [0117] and in [0118] of WO 2008/074548(AGFA GRAPHICS).

The white pigment is preferably present in the range of 9 to 40 wt %,more preferably in the range of 12 to 35% by weight and most preferablyin the range of 15 to 25% by weight, the weight percentage wt % based onthe total weight of the inkjet ink. An amount of less than 9% by weightcannot achieve sufficient covering power and usually exhibits very poorstorage stability and ejection property.

In the most preferred embodiment, the UV curable white inkjet inkpreferably contains a titanium dioxide pigment having an averageparticle size larger than 180 nm. Titanium dioxide pigments with anaverage particle size above 180 nm have a strong opacifying capabilitycompared to other inorganic white pigments, such as calcium carbonatehaving the same average particle size.

The average particle size of the inorganic pigments in the inkjet inkcan be determined in the same manner as explained above for the organiccolour pigments.

Dispersants

Typical polymeric dispersants are copolymers of two monomers but maycontain three, four, five or even more monomers. The properties ofpolymeric dispersants depend on both the nature of the monomers andtheir distribution in the polymer. Copolymeric dispersants preferablyhave the following polymer compositions:

-   -   statistically polymerized monomers (e.g. monomers A and B        polymerized into ABBAABAB);    -   alternating polymerized monomers (e.g. monomers A and B        polymerized into ABABABAB);    -   gradient (tapered) polymerized monomers (e.g. monomers A and B        polymerized into AAABAABBABBB);    -   block copolymers (e.g. monomers A and B polymerized into        AAAAABBBBBB) wherein the block length of each of the blocks (2,        3, 4, 5 or even more) is important for the dispersion capability        of the polymeric dispersant;    -   graft copolymers (graft copolymers consist of a polymeric        backbone with polymeric side chains attached to the backbone);        and    -   mixed forms of these polymers, e.g. blocky gradient copolymers.

Suitable polymeric dispersants are listed in the section on“Dispersants”, more specifically [0064] to [0070] and [0074] to [0077],in EP 1911814 A (AGFA) incorporated herein as a specific reference.

The polymeric dispersant has preferably a number average molecularweight Mn between 500 and 30000, more preferably between 1500 and 10000.

The polymeric dispersant has preferably a weight average molecularweight Mw smaller than 100,000, more preferably smaller than 50,000 andmost preferably smaller than 30,000.

The polymeric dispersant has preferably a polydispersity PD smaller than2, more preferably smaller than 1.75 and most preferably smaller than1.5.

Commercial examples of polymeric dispersants are the following:

-   -   DISPERBYK™ dispersants available from BYK CHEMIE GMBH;    -   SOLSPERSE™ dispersants available from NOVEON;    -   TEGO™ DISPERS™ dispersants from EVONIK;    -   EDAPLAN™ dispersants from MÜNZING CHEMIE;    -   ETHACRYL™ dispersants from LYONDELL;    -   GANEX™ dispersants from ISP;    -   DISPEX™ and EFKA™ dispersants from CIBA SPECIALTY CHEMICALS INC;    -   DISPONER™ dispersants from DEUCHEM; and    -   JONCRYL™ dispersants from JOHNSON POLYMER.

Particularly preferred polymeric dispersants include Solsperse™dispersants from NOVEON, Efka™ dispersants from CIBA SPECIALTY CHEMICALSINC and Disperbyk™ dispersants from BYK CHEMIE GMBH. Particularlypreferred dispersants are Solsperse™ 32000, 35000 and 39000 dispersantsfrom NOVEON.

The polymeric dispersant is preferably used in an amount of 2 to 200 wt%, more preferably 10 to 120 wt %, most preferably 30 to 80 wt % basedon the weight of the pigment.

Photoinitiators

The liquid UV curable inkjet ink contains one or more photoinitiators,preferably one or more free radical photoinitiators. A free radicalphotoinitiator is a chemical compound that initiates polymerization ofmonomers and oligomers when exposed to actinic radiation by theformation of a free radical.

Two types of free radical photoinitiators can be distinguished and usedin the liquid UV curable inkjet ink. A Norrish Type I initiator is aninitiator which cleaves after excitation, yielding the initiatingradical immediately. A Norrish type II-initiator is a photoinitiatorwhich is activated by actinic radiation and forms free radicals byhydrogen abstraction from a second compound that becomes the actualinitiating free radical. This second compound is called a polymerizationsynergist or co-initiator. Both type I and type II photoinitiators canbe used in the present invention, alone or in combination.

In order to increase the photosensitivity further, the liquid UV curableinkjet ink may additionally contain co-initiators. Suitable examples ofco-initiators can be categorized in three groups:

(1) tertiary aliphatic amines such as methyldiethanolamine,dimethylethanolamine, triethanolamine, triethylamine andN-methylmorpholine;(2) aromatic amines such as amylparadimethylaminobenzoate,2-n-butoxyethyl-4-(dimethylamino) benzoate,2-(dimethylamino)ethylbenzoate, ethyl-4-(dimethylamino)benzoate, and2-ethylhexyl-4-(dimethylamino)benzoate; and(3) (meth)acrylated amines such as dialkylamino alkyl(meth)acrylates(e.g., diethylaminoethylacrylate) or N-morpholinoalkyl-(meth)acrylates(e.g., N-morpholinoethyl-acrylate).The preferred co-initiators are aminobenzoates.

Suitable photo-initiators are disclosed in CRIVELLO, J. V., et al.VOLUME III: Photoinitiators for Free Radical Cationic. 2nd edition.Edited by BRADLEY, G. London, UK: John Wiley and Sons Ltd, 1998. p.287-294.

Specific examples of photo-initiators may include, but are not limitedto, the following compounds or combinations thereof: benzophenone andsubstituted benzophenones, 1-hydroxycyclohexyl phenyl ketone,thioxanthones such as isopropylthioxanthone,2-hydroxy-2-methyl-1-phenylpropan-1-one,2-benzyl-2-dimethylamino-(4-morpholinophenyl) butan-1-one, benzildimethylketal, bis (2,6-dimethylbenzoyl)-2,4,4-trimethylpentylphosphineoxide, 2,4,6trimethylbenzoyldiphenylphosphine oxide,2-methyl-1-[4-(methylthio) phenyl]-2-morpholinopropan-1-one,2,2-dimethoxy-1,2-diphenylethan-1-one or 5,7-diiodo-3-butoxy-6-fluorone.

Suitable commercial photo-initiators include Irgacure™ 184, Irgacure™500, Irgacure™ 907, Irgacure™ 369, Irgacure™ 1700, Irgacure™ 651,Irgacure™ 819, Irgacure™ 1000, Irgacure™ 1300, Irgacure™ 1870, Darocur™1173, Darocur™ 2959, Darocur™ 4265 and Darocur™ ITX available from CIBASPECIALTY CHEMICALS, Lucerin™ TPO available from BASF AG, Esacure™KT046, Esacure™ KIP150, Esacure™ KT37 and Esacure™ EDB available fromLAMBERTI, H-Nu™ 470 and H-Nu™ 470X available from SPECTRA GROUP Ltd.

In a particularly preferred embodiment of the liquid UV curable inkjetink, the one or more photoinitiators include an acylphosphine oxidephotoinitiator and a thioxanthone photoinitiator. Such a combinationallows for fast UV curing with UV LEDS emitting above 370 nm.

In a preferred embodiment, at least one of the one or morephotoinitiators is selected from the group consisting of a polymericphotoinitiator, a polymerizable photoinitiator and a photoinitiatorcontaining a plurality of photoinitiating groups, more preferablyselected from the group consisting of a polymeric photoinitiator and apolymerizable photoinitiator. Such a diffusion hindered photoinitiatorexhibits a much lower mobility in a cured layer of the liquid UV curableinkjet ink than a low molecular weight monofunctional photoinitiator,such as benzophenone. Including diffusion hindered photoinitiators, andalso diffusion hindered co-initiators do not only have a safetyadvantage for the operator of the inkjet printer, but are alsoenvironmentally friendly as these compounds cannot be leached out e.g.from an outdoor billboard by acid rain.

Most preferably the diffusion hindered photoinitiator is a polymerizablephotoinitiator, preferably having at least one acrylate group, morepreferably at least two or three acrylate groups. And most preferablythe diffusion hindered coinitiator is a polymerizable coinitiator,preferably having at least one acrylate group.

Suitable diffusion hindered photoinitiator may contain one or morephotoinitiating functional groups derived from a Norrish typeI-photoinitiator selected from the group consisting of benzoinethers,benzil ketals, α,α-dialkoxyacetophenones, α-hydroxyalkylphenones,α-aminoalkylphenones, acylphosphine oxides, acylphosphine sulphides,α-haloketones, α-halosulfones and phenylglyoxalates.

A suitable diffusion hindered photoinitiator may contain one or morephotoinitiating functional groups derived from a Norrish typeII-initiator selected from the group consisting of benzophenones,thioxanthones, 1,2-diketones and anthraquinones.

Suitable diffusion hindered photoinitiators are also those disclosed inEP 2053101 A (AGFA GRAPHICS) in paragraphs [0074] and [0075] fordifunctional and multifunctional photoinitiators, in paragraphs [0077]to [0080] for polymeric photoinitiators and in paragraphs [0081] to[0083] for polymerizable photoinitiators.

Other preferred polymerizable photoinitiators are those disclosed in EP2065362 A (AGFA) and EP 2161264 A (AGFA), incorporated herein byreference.

In a particularly preferred embodiment of the liquid UV curable inkjetink, the one or more photoinitiators include an acylphosphine oxidephotoinitiator and a polymeric or polymerizable thioxanthonephotoinitiator. Such a combination allows for both the safety advantage,e.g. food packaging, and the fast UV curing with UV LEDS emitting above370 nm. The acylphosphine oxide photoinitiator may be a bisacylphosphineoxide or a polymeric or polymerizable (bis)acylphosphineoxidephotoinitiator.

Suitable polymeric and polymerizable acylphosphineoxide photoinitiatorsare disclosed in US 2015344711 A (FUJIFILM), WO 2015/031927 A (DURST)and US 2015197651 A (FUJIFILM).

A preferred amount of photoinitiator is 0-50 wt %, more preferably0.1-20 wt %, and most preferably 0.3-15 wt % of the total weight of theliquid UV curable inkjet ink. In the most preferred embodiment, theamount of photoinitiator is at least 7.0 wt % or 8.0 wt % of the totalweight of the liquid UV curable inkjet ink for obtaining high curingspeed.

Preferred diffusion hindered co-initiators are the polymerizableco-initiators disclosed in EP 2053101 A (AGFA GRAPHICS) in paragraphs[0088] and [0097].

Preferred diffusion hindered co-initiators include a polymericco-initiator having a dendritic polymeric architecture, more preferablya hyperbranched polymeric architecture. Preferred hyperbranchedpolymeric co-initiators are those disclosed in US 2006014848 (AGFA)incorporated herein as a specific reference.

The liquid UV curable inkjet ink preferably comprises the diffusionhindered co-initiator in an amount of 0.1 to 50 wt %, more preferably inan amount of 0.5 to 25 wt %, most preferably in an amount of 1 to 10 wt% of the total weight of the liquid UV curable inkjet ink.

Monofunctional Polymerizable Compounds

The polymerizable composition of the liquid UV curable inkjet inkaccording to a preferred embodiment of the present invention contains atleast one monofunctional polymerizable compound. A monofunctionalpolymerizable compound contains a single polymerizable group, preferablya free radical polymerizable group selected from the group consisting ofan acrylate, a methacrylate, an acrylamide, a methacrylamide, a styrenegroup, a maleate, a fumarate, an itaconate, a vinyl ether, a vinylester, an allyl ether and an allyl ester.

Any monofunctional polymerizable compound commonly known in the art maybe employed. A combination of monomers and oligomers may be used. Themonofunctional polymerizable compounds may be any monomer and/oroligomer found in the Polymer Handbook Vol 1+2, 4th edition, edited byJ. BRANDRUP et al., Wiley-Interscience, 1999. An oligomer in the presentinvention is understood to contain 2 to 8 repeating monomeric units.

In a preferred embodiment, the monofunctional polymerizable compoundsare selected from acrylic acid, methacrylic acid, maleic acid (or theresalts), maleic anhydride, alkyl(meth)acrylates (linear, branched andcycloalkyl) such as methyl(meth)acrylate, n-butyl(meth)acrylate,tert-butyl(meth)acrylate, cyclohexyl(meth)acrylate, and2-ethylhexyl(meth)acrylate; aryl(meth)acrylates such asbenzyl(meth)acrylate, and phenyl(meth)acrylate;hydroxyalkyl(meth)acrylates such as hydroxyethyl(meth)acrylate, andhydroxypropyl(meth)acrylate; (meth)acrylates with other types offunctionalities (e.g. oxiranes, amino, fluoro, polyethylene oxide,phosphate substituted) such as glycidyl (meth)acrylate,dimethylaminoethyl(meth)acrylate, trifluoroethyl acrylate,methoxypolyethyleneglycol (meth)acrylate, and tripropyleneglycol(meth)acrylate phosphate; allyl derivatives such as allyl glycidylether; styrenics such as styrene, 4-methylstyrene, 4-hydroxystyrene,4-acetostyrene, and styrenesulfonic acid; (meth)acrylonitrile;(meth)acrylamides (including N-mono and N,N-disubstituted) such asN-benzyl (meth)acrylamide; maleimides such as N-phenyl maleimide; vinylderivatives such as vinylcaprolactam, vinylpyrrolidone, vinylimidazole,vinylnapthalene, and vinyl halides; vinylethers such as vinylmethylether; vinylesters of carboxylic acids such as vinylacetate,vinylbutyrate, and vinyl benzoate. In a more preferred embodiment, themonofunctional polymerizable compounds are selected from monoacrylatesand vinyllactams, such as N-vinylcaprolactam. Particularly preferredmonofunctional polymerizable compounds are selected from the groupconsisting of isoamyl acrylate, stearyl acrylate, lauryl acrylate, octylacrylate, decyl acrylate, isoamyl acrylate, isostearyl acrylate,2-ethylhexyl-diglycol acrylate, 2-hydroxybutyl acrylate,2-acryloyloxyethylhexahydrophthalic acid, butoxyethyl acrylate,ethoxydiethylene glycol acrylate, methoxydiethylene glycol acrylate,methoxypolyethylene glycol acrylate, methoxypropylene glycol acrylate,phenoxyethyl acrylate, tetrahydrofurfuryl acrylate, isobornyl acrylate,2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate,2-hydroxy-3-phenoxypropyl acrylate, vinyl ether acrylate,2-acryloyloxyethylsuccinic acid, 2-acryloyxyethylphthalic acid,2-acryloxyethyl-2-hydroxyethyl-phthalic acid, lactone modified flexibleacrylate, t-butylcyclohexyl acrylate, caprolactone acrylate, cyclictrimethylolpropane formal acrylate, cyclic trimethylolpropane formalacrylate, ethoxylated nonyl phenol acrylate, isodecyl acrylate, isooctylacrylate, octyldecyl acrylate, alkoxylated phenol acrylate, tridecylacrylate and acryloylmorpholine

In a more preferred embodiment, the monofunctional polymerizablecompounds are selected from monoacrylates and vinyllactams, such asN-vinylcaprolactam.

The N-vinyllactam is preferably a compound represented by Formula (I):

wherein n denotes an integer of 2 to 6; n is preferably an integer of 3to 5 from the viewpoint of flexibility after the ink composition iscured, adhesion to a substrate, and ready availability of startingmaterials, n is more preferably 3 or 5, and n is particularly preferably5, which is N-vinylcaprolactam. N-vinylcaprolactam is preferable sinceit is readily available at a relatively low price, and givesparticularly good ink curability and adhesion of a cured film to arecording medium.

The N-vinyllactam may have a substituent such as an alkyl group or anaryl group on the lactam ring, and may have a saturated or unsaturatedring structure bonded to the lactam ring. The compound represented byFormula (a) may be used singly or in a combination of two or morecompounds.

For certain applications preferably no monofunctional (meth)acrylatesare employed. For example, when the substrate is a textile that is worndirectly on the human skin it may give rise to skin sensitization. Insuch a case, the monomers and oligomers are preferably selected from agroup comprising or consisting of vinyls, acrylamides, methacrylamides,vinyl carbonates, vinyl ethers, vinyl esters, vinyl carbamates, allylethers, allyl esters and their corresponding alkyne compounds.Particularly preferred are polymerizable compounds including an allylether group, vinyl carbonate group and alkyne group.

Polyfunctional Polymerizable Compounds

The polymerizable composition of the liquid UV curable inkjet inkaccording to a preferred embodiment of the present invention contains atleast one polyfunctional polymerizable compound. A polyfunctionalpolymerizable compound contains two, three or more polymerizable groups,preferably free radical polymerizable groups selected from the groupconsisting of an acrylate, a methacrylate, an acrylamide, amethacrylamide, a styrene group, a maleate, a fumarate, an itaconate, avinyl ether, a vinyl ester, an allyl ether and an allyl ester.

Any polyfunctional polymerizable compound commonly known in the art maybe employed. A combination of monomers and oligomers may be used. Thepolyfunctional polymerizable compounds may be any monomer and/oroligomer found in the Polymer Handbook Vol 1+2, 4th edition, edited byJ. BRANDRUP et al., Wiley-Interscience, 1999. An oligomer in the presentinvention is understood to contain 2 to 8 repeating monomeric units.

In a preferred embodiment, the polyfunctional polymerizable compound isa duofunctional acrylate containing two polymerizable groups, namely twoacrylate groups.

Preferred polyfunctional acrylates include triethylene glycoldiacrylate, tetraethylene glycol diacrylate, polyethylene glycoldiacrylate, dipropylene glycol diacrylate, tripropylene glycoldiacrylate, polypropylene glycol diacrylate, 1,4-butanediol diacrylate,1,6-hexanediol diacrylate, 1,9-nonanediol diacrylate, neopentyl glycoldiacrylate, dimethyloltricyclodecane diacrylate, bisphenol A EO(ethylene oxide) adduct diacrylate, bisphenol A PO (propylene oxide)adduct diacrylate, hydroxypivalate neopentyl glycol diacrylate,propoxylated neopentyl glycol diacrylate, alkoxylateddimethyloltricyclodecane diacrylate and polytetramethylene glycoldiacrylate, trimethylolpropane triacrylate, EO modifiedtrimethylolpropane triacrylate, tri (propylene glycol) triacrylate,caprolactone modified trimethylolpropane triacrylate, pentaerythritoltriacrylate, pentaerythritol tetraacrylate, pentaerythritolethoxytetraacrylate, dipentaerythritol hexaacrylate, ditrimethylolpropanetetraacrylate, glycerolpropoxy triacrylate, and caprolactam modifieddipentaerythritol hexaacrylate

Other suitable difunctional acrylates include alkoxylated cyclohexanonedimethanol diacrylate, alkoxylated hexanediol diacrylate, dioxane glycoldiacrylate, dioxane glycol diacrylate, cyclohexanone dimethanoldiacrylate, diethylene glycol diacrylate and neopentyl glycoldiacrylate.

Other polyfunctional acrylates include propoxylated glycerinetriacrylate and propoxylated trimethylolpropane triacrylate,di-trimethylolpropane tetraacrylate, dipentaerythritol pentaacrylate,ethoxylated pentaerythritol tetraacrylate, methoxylated glycol acrylatesand acrylate esters

Preferred polyfunctional acrylates include dipropylene glycoldiacrylate, tripropylene glycol diacrylate, 1,6-hexanediol diacrylate,cyclohexanone dimethanol diacrylate, polyethyleneglycol 200 diacrylate,3-methyl 1,5-pentanediol diacrylate, pentaerythritol tetraacrylate,trimethylolpropane triacrylate and dipentaerythritol pentaacrylate.

The polyfunctional polymerizable compound may have two differentpolymerizable groups, such as a vinylether group and an acrylate group.Preferred vinylether acrylates are those disclosed in U.S. Pat. No.6,310,115 (AGFA). A particularly preferred compound is2-(2-vinyloxyethoxy)ethyl acrylate. Other suitable vinylether acrylatesare those disclosed in columns 3 and 4 of U.S. Ser. No. 67/679,890 B(NIPPON SHOKUBAI).

Instead of difunctional or polyfunctional acrylates, also theirmethacrylate analogues may be used.

For certain applications preferably no polyfunctional (meth)acrylatesare employed. For example, when the substrate is a textile that is worndirectly on the human skin it may give rise to skin sensitization.

A preferred alternative free radical curing chemistry is the so-calledthiol-ene and thiol-yne chemistry. In such a chemistry, a combination ofat least one polyfunctional thiol compound and at least onepolyfunctional polymerizable compound is used. The polyfunctionalpolymerizable compound is a polyfunctional monomers or oligomer having aplurality of polymerizable groups selected from a group consisting of avinyl group, an acrylamide group, a methacrylamide group, a vinylcarbonate group, a vinyl ether group, a vinyl ester group, a vinylcarbamate group, an allyl ether groups, an allyl ester group and analkyne group. Particularly preferred are polymerizable compoundsincluding allyl ether groups, vinyl carbonate groups and alkyne groups.

Synthesis of such monomers is disclosed in the relevant literature, forexample in HURD, Charles D. Vinylation and the Formation of Acylals.Journal Am. Chem. Soc. 1956, vol. 78, no. 1, p. 104_106.; LOBELL, M., etal. Synthesis of hydroxycarboxylic acid vinyl esters. MP Synthesis.1994, vol. 4, p. 375-377.; LEE, T. Y., et al. Synthesis, Initiation, andPolymerization of Photoinitiating Monomer. Macromolecules. 2005, vol.38, no. 18, p. 7529-7531.; ATTA, A.M., et al. New vinyl ester resinsbased on rosin for coating applications. React. Funct. Polym. 2006, vol.66, p. 1596-1608.; WO 01/00634 A (WRIGHT CHEM CORP); and ROHR, Markus,et al. Solvent-free ruthenium-catalysed vinylcarbamate synthesis fromphenylacetylene and diethylamine in ‘supercritical’ carbon dioxide.Green Chemistry. 2001, vol. 3, p. 123-125.

Preferred polymerizable oligomers and polymers are urethanes,polyesters, polyethers, polycarbonates, poly-carbamates, polyureas andstraight-chain oligomers having the following polymerizable groups:acrylate, methacrylate, vinyl, acrylamide, methacrylamide, vinylcarbonate, vinyl ether, vinylester-vinyl carbamate groups, as well astheir corresponding alkyne compounds.

Particularly preferred monomers are selected from the group consistingof di- or oligofunctional allylethers, di- or oligofunctional allylesters, di- or oligofunctional vinyl ethers, di- or oligofunctionalvinyl esters and di- or oligofunctional norbornene derivatives. Typicalallyl ethers can be selected from pentaerythritol tetraallyl ether,glycerol triallyl ether, 1,6-hexane diol diallyl ether, cyclohexanedimethanol diallyl ether, trimethylolpropane triallyl ether,dipentaerythritol hexaallyl ether and ethoxylated and propoxylatedderivatives thereof. Typical vinylethers can be selected frompentaerythritol tetravinyl ether, glycerol trivinyl ether, 1,6-hexanediol divinyl ether, cyclohexane dimethanol divinyl ether,trimethylolpropane trivinyl ether, dipentaerythritol hexavinyl ether andethoxylated and propoxylated derivatives thereof. Typical allyl esterscan be selected from adipic acid diallyl ester, terephtalic acid diallylester, trimellitic acid triallyl ester, pyromellitic acid tetraallylester, citric acid triallyl ester and glutaric acid diallyl ester.Typical vinyl esters can be selected from adipic acid divinyl ester,terephtalic acid divinyl ester, trimellitic acid trivinyl ester,pyromellitic acid tetravinyl ester, citric acid trivinyl ester andglutaric acid divinyl ester.

Thiol-yne chemistry has been described as an extension for thiol-enechemistry to design cross-linked networks with a higher cross-linkingdensity and glass transition temperature in comparison with thiol-enebased networks. The chemistry has recently been reviewed by Lowe et al.(Journal of Materials Chemistry, 20, 4745-4750 (2010)) and by HoogenboomR. (Angew. Chem. Int. Ed. 49, 3415-3417 (2010)).

Optionally photochemically induced radical double addition ofpolyfunctional thiol compounds to di- or multifunctional alkynes is thebasis of thiol-yne chemistry. In principle any di- or multifunctionalalkyne, including polymeric alkynes, can be used in combination with anydi- or polyfunctional thiol compound.

In a preferred embodiment, at least one of the alkyne functions in thedi- or polyfunctional alkynes is represented by H—C≡C—*, whererepresents the covalent bond to the rest of the di- or polyfunctionalalkyne.

In a more preferred embodiment, all of the alkyne groups in the di- orpolyfunctional alkyne are represented by H—C≡C—*.

In an even more preferred embodiment, the alkyne functions in said di-or polyfunctional alkyne is selected from the group consisting of apropargyl ether, a propargyl ester, a propargyl urethane, a propargylureum, a propargyl carbonate, a propargyl amide, a propargyl thioetherand a propargyl amine. In a further preferred embodiment, said alkynegroup is selected from the group consisting of a propargyl ether, apropargyl ester and propargyl urethane, a propargyl ester and apropargyl urethane being particularly preferred.

Preferred thiol compounds for conducting thiol-ene or thiol-ynechemistry are thiol molecules including at least two thiol groups.Preferred thiol molecules include two to six thiol groups, preferablythree to five thiol groups, and most preferably four thiol groups.

The thiol molecule is preferably a compound comprising an aliphaticthiol.

In a preferred embodiment, the thiol molecule is represented by Formula(I):

wherein n represents an integer from 1 to 4; m represents an integerfrom 2 to 6; and R represents an m-valent linking group comprising atmost 20 carbon atoms.

In a preferred embodiment n represents 1 or 2.

In a preferred embodiment m represents 3 or 4.

In a more preferred embodiment n represents 1 or 2 and m represents aninteger from 2 to 6. In the most preferred embodiment n represents 1 or2 and m represents 3 or 4.

In a preferred embodiment, the thiol compound has a molecular weightsmaller than 1,000 Dalton, more preferably the thiol compound has amolecular weight smaller than 500 Dalton.

Particularly preferred primary thiol molecules include tetra(ethyleneglycol) dithiol (CAS 2781-02-4), glycol di(3-mercaptopropionate) (CAS22504-50-3), glyceryl dithioglycolate (CAS 63657-12-5), glycoldimercaptoacetate (CAS 123-81-9), trimethylolpropane trimercaptoacetate(CAS 10193-96-1), pentaerythritol tetramercaptoacetate (CAS 10193-99-4),glycol di(3-mercaptopropionate) (CAS 22504-50-3), trimethylolpropanetri(3-mercaptopropionate) (CAS 33007-83-9), pentaerythritoltetra(3-mercaptopropionate) (CAS 7575-23-7), dipentaerythritolhexa(3-mercaptopropionate) (CAS 25359-71-1),ethoxylated-trimethylolpropane tri-3-mercaptopropionate (CAS345352-19-4), and tris[2-(3-mercaptopropionyloxy)ethyl]isocyanurate (CAS36196-44-8).

The above and other thiol molecules are commercially available, e.g. asThiocure™ grades from Bruno Bock Chemische Fabrik GmbH & Co. KG.

Suitable thiol molecules include 1,1,1-trimethylolpropanetris(3-mercaptopropyl)ether, 1,2,4-tris(2-mercaptoethyl)cyclohexane,tri(3-mercaptopropyl) trimetylolpropane and others disclosed by WO2011/004255 A (KUROS BIOSURGERY).

It was found that thiol molecules having secondary thiol groupsexhibited less odour than thiol molecules having only primary thiolgroups. Hence, the thiol molecule preferably includes at least twosecondary thiol groups, more preferably the thiol molecule includes twoto six secondary thiol groups, preferably three to five secondary thiolgroups, and most preferably four secondary thiol groups.

A particularly preferred thiol molecule having secondary thiol groups ispentaerythritol tetrakis (3-mercaptobutylate). The latter is availableas Omnimer™ PE1 from IGM RESINS and Karenz MT™ PE1 from SHOWA DENKO.

For minimizing odour of the liquid UV curable inkjet ink, especiallyafter UV curing, the molar ratio of thiol molecules having primary thiolgroups over thiol compounds having at least one secondary thiol group ispreferably 0 to 4, more preferably the molar ratio is 0, meaning thatthe thiol molecules in the liquid UV curable inkjet ink consist of thiolmolecules containing at least one secondary thiol group. For calculatingthe molar ratio, a thiol molecule having primary thiol groups isconsidered to have only primary thiol groups, while thiol moleculescontaining at least one secondary thiol group may also include primarythiol groups.

In the most preferred embodiment, the thiol molecules consist of thiolmolecules containing only secondary thiol groups.

For improving mechanical performance and limited potential for wateruptake, leachables and degradation, the thiol molecules are preferablyester-free thiol molecules.

Particularly preferred ester-free thiol molecules are silane based thiolmolecules and siloxane based thiol molecules. Such compounds can easilybe synthesized by reacting thioacetic acid with functional alkenes togive thioester derivatives that can be hydrolyzed under alkaline oracidic conditions.

Suitable silane based thiol molecules and siloxane based thiol moleculesare disclosed by WO 2011/004255 A (KUROS BIOSURGERY), especially thosein the examples 1 to 6.

A preferred example of a silane based thiol molecule for use in theliquid UV curable inkjet ink is tetra(3-mercaptopropyl)silane, whichsynthesis is described in Example 5 of WO 2011/004255 A (KUROSBIOSURGERY).

A preferred example of a siloxane based thiol molecule for use in theliquid UV curable inkjet ink is2,4,6,8-tetra(2-mercaptoethyl)-2,4,6,8-tetramethylcyclotetrasiloxane,which synthesis is described in Example 4 of WO 2011/004255 A (KUROSBIOSURGERY).

More preferably silane based thiol molecules and siloxane based thiolmolecules including secondary thiol groups are used in the liquid UVcurable inkjet ink according to the invention. Such thiol molecules notonly improve mechanical properties, but also reduce the odour problem.

A preferred example of a silane based thiol molecule containingsecondary thiol groups is the compound represented by the formula TH-1:

The synthesis of TH-1 may be performed in a multi-step reaction. In thefirst step, hydrogen bromide is reacted with tetraallylsilane to givetetrakis(2-bromopropyl)silane. The latter is converted with thiourea toits isothiouronium salt, which is then hydrolyzed with aqueous sodiumhydroxide to give TH-1.

The thiol compound may also be a so-called thiol pigment. A thiolpigment is an inorganic pigment, such as a silica pigment or atitaniumdioxide pigment, which surface has been functionalized with twoor more thiol groups. The main advantage is that liquid UV curableinkjet inks containing a thiol pigment exhibit no or minor odour priorto UV curing, which is generally not the case for liquid UV curableinkjet inks containing polyfunctional thiol molecules. The latter causea bad odour even at small amounts of evaporated thiol molecules.

Silica nanoparticles are preferred because they are usually small-sized,monodisperse and can be easily surface-modified. A monodispersedistribution is advantageous for the transparency of printed colourinks, thus enlarging the colour gamut.

Thiol groups are preferably introduced on the surface using analkoxysilane containing a thiol group. Typical examples of siloxanescontaining a thiol are 3-mercapotopropyl triethoxysilane,3-mercaptopropyl trimethoxysilane, 2-mercaptoethyl triethoxysilane,4-mercaptobutyl triethoxysilane, 2-mercaptopropyl trimethoxysilane and3-mercaptobutyl trimethoxysilane. A preferred alkoxysilane containing athiol group is 3-mercaptopropyl trimethoxysilane (MPTMS).

An example of a suitable synthesis scheme for a thiol pigment is asfollows: a dry phase deposition method was used to functionalize silicaparticles (e.g. Ludox™ TM-50 from GRACE having an average particle sizeof about 22 nm). The silica particles were dispersed in anhydrousethanol (15 mL of ethanol per gram of silica) and MPTMS (available fromALDRICH) was added such that the ratio of the amount of silica (in g) tothe amount of MPTMS (in mL) was 3:7. Ultra high purity grade nitrogenwas bubbled through the mixture to evaporate the ethanol under fumehood, thus depositing MPTMS on the surface of the silica. For thesilanization reaction, the silica was then placed in oven at 120° C. for9 hours. The material was allowed to cool and washed twice with 50 mL ofanhydrous ethanol to remove any physically adsorbed MPTMS and driedagain in oven. The silica was analyzed using FTIR to verify the MPTMSdeposition on the silica surface.

The number of thiol groups on the thiol pigment surface can be easilymodified as desired as long as at least two thiol groups are present.However, usually a large number of thiol groups is present on thepigment surface, preferably more than ten thiol groups, more preferablyeven more than twenty or fifty thiol groups.

A commercially available thiol pigment having however a large averageparticle size of 2.2 μm is Aktisil™ MM mercapto modified from HOFMANNMINERAL. The average particle size of the thiol pigment as measuredaccording to ISO 13320-1 is preferably between 10 nm and 1 μm, morepreferably between 15 nm and 250 nm, and most preferably between 20 nmand 150 nm.

Due to its higher molecular weight per unit, it is not necessary toinclude secondary thiol groups for improvement of the odour. In factpreferably primary thiol groups are included because of their greaterreactivity in thiol-ene and thiol-yne click chemistry.

Stabilizers

The liquid UV curable inkjet ink may contain a polymerization inhibitor.Suitable polymerization inhibitors include phenol type antioxidants,hindered amine light stabilizers, phosphor type antioxidants,hydroquinone monomethyl ether commonly used in (meth)acrylate monomers,and hydroquinone, t-butylcatechol, pyrogallol may also be used.

Suitable commercial inhibitors are, for example, Sumilizer™ GA-80,Sumilizer™ GM and Sumilizer™ GS produced by Sumitomo Chemical Co. Ltd.;Genorad™ 16, Genorad™ 18 and Genorad™ 20 from Rahn AG; Irgastab™ UV10and Irgastab™ UV22, Tinuvin™ 460 and CGS20 from Ciba SpecialtyChemicals; Floorstab™ UV range (UV-1, UV-2, UV-5 and UV-8) fromKromachem Ltd, Additol™ S range (S100, S110, S120 and S130) from CytecSurface Specialties.

A preferred polymerization inhibitor is Irgastab™ UV10 from BASF.

In a preferred embodiment, the polymerization inhibitor is a mixture ofdifferent types of polymerization inhibitors. Preferred polymerizationinhibitors are mixtures of an oxyl free radical-based polymerizationinhibitor, a phenol-based polymerization inhibitor, and an amine-basedpolymerization inhibitor. Suitable examples are given in EP 2851402 A(FUJIFILM).

Since excessive addition of these polymerization inhibitors will lowerthe ink sensitivity to curing, it is preferred that the amount capableof preventing polymerization is determined prior to blending. The amountof a polymerization inhibitor is preferably lower than 2 wt % based onthe total weight of the liquid UV curable inkjet ink.

Surfactants

Surfactants may used in the liquid UV curable inkjet ink reduce thesurface tension in order to improve the spreading of the inkjet ink. Aliquid UV curable inkjet ink must meet stringent performance criteria inorder to be adequately jettable with high precision, reliability andduring an extended period of time. The surface tension is not onlydetermined by the amount and type of surfactant, but also by thepolymerizable compounds and other additives in the ink composition.

The surfactant(s) can be anionic, cationic, non-ionic, or zwitter-ionicand are preferably added in a total quantity of no more than 2 wt %,preferably less than 1 wt % based on the total weight of the liquid UVcurable inkjet ink.

Suitable surfactants include fluorinated surfactants, fatty acid salts,ester salts of a higher alcohol, alkylbenzene sulphonate salts,sulphosuccinate ester salts and phosphate ester salts of a higheralcohol (for example, sodium dodecylbenzenesulphonate and sodiumdioctylsulphosuccinate), ethylene oxide adducts of a higher alcohol,ethylene oxide adducts of an alkylphenol, ethylene oxide adducts of apolyhydric alcohol fatty acid ester, and acetylene glycol and ethyleneoxide adducts thereof (for example, polyoxyethylene nonylphenyl ether,and SURFYNOLTN 104, 104H, 440, 465 and TG available from AIR PRODUCTS &CHEMICALS INC.).

Preferred surfactants include fluoro surfactants (such as fluorinatedhydrocarbons) and silicone surfactants. The silicones are typicallysiloxanes and can be alkoxylated, polyether modified, polyestermodified, polyether modified hydroxy functional, amine modified, epoxymodified and other modifications or combinations thereof. Preferredsiloxanes are polymeric, for example polydimethylsiloxanes.

The fluorinated or silicone compound used as a surfactant may be apolymerizable surfactant. Suitable polymerizable compounds havingsurface-active effects include, for example, polyacrylate copolymers,silicone modified acrylates, silicone modified methacrylates, acrylatedsiloxanes, polyether modified acrylic modified siloxanes, fluorinatedacrylates, and fluorinated methacrylate. These acrylates can be mono-,di-, tri- or higher functional (meth)acrylates.

Depending upon the application a surfactant can be used with a high, lowor intermediate dynamic surface tension. Silicone surfactants aregenerally known to have low dynamic surface tensions while fluorinatedsurfactants are known to have higher dynamic surface tensions.

Silicone surfactants are preferred in the liquid UV curable inkjet inkof the present invention, especially the reactive silicone surfactants,which are able to be polymerized together with the polymerizablecompounds during the curing step.

Examples of useful commercial silicone surfactants are those supplied byBYK CHEMIE GMBH (including Byk™-302, 307, 310, 331, 333, 341, 345, 346,347, 348, UV3500, UV3510 and UV3530), those supplied by TEGO CHEMIESERVICE (including Tego Rad™ 2100, 2200N, 2250, 2300, 2500, 2600 and2700), Ebecryl™ 1360 a polysilixone hexaacrylate from CYTEC INDUSTRIESBV and Efka™-3000 series (including Efka™-3232 and Efka™-3883) from EFKACHEMICALS B.V.; and those supplied by MOMENTIVE PERFORMANCE MATERIALS,such as Coatasil™ 7500.

Preparation of Inkjet Inks

The preparation of pigmented UV curable inkjet inks is well-known to theskilled person. Preferred methods of preparation are disclosed inparagraphs [0076] to [0085] of WO 2011/069943 (AGFA).

Inkjet Printing Methods

A UV curable inkjet printing method according to a preferred embodimentof the present invention includes the steps of: a) jetting one or moreliquid UV curable inkjet inks as described above with a print head at ajetting temperature of 45° C. or more on a substrate; and b) UV curingthe jetted liquid UV curable inkjet ink on the substrate.

The one or more liquid UV curable inkjet inks are preferably at atemperature between 45° C. and 65° C., more preferably not higher than55° C. or 60° C. Above 65° C. the reliability of the liquid UV curableinkjet ink is reduced as undesired ‘dark’ polymerization may occur, i.e.polymerization not caused by UV radiation but by remaining an extendedtime at high temperature.

The print head is preferably a through-flow piezoelectric drop-on-demandprint head. For limiting the time that the liquid UV curable inkjet inkremains at higher temperatures, the liquid UV curable inkjet ink may beheated to temperatures to the jetting temperature just prior to enteringthe through-flow print head, while upon exiting the through-flow printhead unused ink may be cooled down to 45° C. or less. In this way, themajor part of the ink circulation circuit of the ink supply to thethrough-flow piezoelectric DOD print head is kept at a temperaturepreventing undesired ‘dark’ polymerization of the inkjet ink.

The UV curable inkjet printing method preferably makes use of UV LEDcuring that is performed between 100 ms to 1 s, more preferably between400 to 600 ms after the liquid UV curable inkjet ink was jetted on thesubstrate.

In a preferred embodiment, the average ink layer thickness to reach thetarget density of the UV cured one or more liquid UV curable inkjet inkis smaller than 8 μm, preferably smaller than 6 μm and most preferablysmaller than 3 or 4 μm.

Printing Devices

The liquid UV curable inkjet ink is jetted by one or more print headsejecting small droplets in a controlled manner through nozzles onto asubstrate moving relative to the print head(s).

A preferred print head for the inkjet printing system is a piezoelectrichead. Piezoelectric inkjet printing is based on the movement of apiezoelectric ceramic transducer when a voltage is applied thereto. Theapplication of a voltage changes the shape of the piezoelectric ceramictransducer in the print head creating a void, which is then filled withinkjet ink or liquid. When the voltage is again removed, the ceramicexpands to its original shape, ejecting a drop of ink from the printhead.

A preferred piezoelectric print head is a so called push mode typepiezoelectric print head, which has a rather large piezo-element capableof ejecting larger or high viscous inkjet ink droplets. Such a printhead is available from RICOH as the GEN5s print head.

A preferred piezoelectric print head is a so-called through-flowpiezoelectric drop-on-demand print head. Such a print head is availablefrom TOSHIBA TEC as the CF1ou print head.

However the inkjet printing method according to the present invention isnot restricted to piezoelectric inkjet printing. Other inkjet printheads can be used and include various types, such as a continuous typeprint head.

The inkjet print head normally scans back and forth in a transversaldirection across the moving ink-receiver surface. Often the inkjet printhead does not print on the way back. Bi-directional printing ispreferred for obtaining a high areal throughput.

Another preferred printing method is by a “single pass printingprocess”, which can be performed by using page wide inkjet print headsor multiple staggered inkjet print heads which cover the entire width ofthe ink-receiver surface. In a single pass printing process the inkjetprint heads usually remain stationary and the ink-receiver surface istransported under the inkjet print heads.

In a particularly preferred embodiment, the inkjet printing of theliquid UV curable inkjet ink is performed in a multi-pass printing mode.Multi-pass printing is a technique used to reduce banding in ink-jetprinting. Dots of ink, when still in liquid form, tend to run togetherdue to surface tension. This is referred to as coalescence. To print ahigh quality image it is important to print individual round dots. Butto achieve full saturated colours, the dots must overlap to completelycover the paper. By only printing a portion of the image data so as toavoid simultaneously printing adjacent dots during each printing cycle,coalescence may be largely avoided. Additionally, by avoiding allhorizontal adjacencies, the transverse speed of the printing mechanismcan be increased up to two times the rated print speed of the printhead. In a preferred embodiment, the number of passes used is to 2 to 6passes, more preferably no more than 4 passes.

An advantage of using a multi-pass printing mode is that the liquid UVcurable inkjet ink is cured in a consecutive passes, rather than in asingle pass which would require a curing device with a high UV output.The print head lifetime is also larger for multi pass printing. While insingle pass printing one side shooter is sufficient to replace the wholeprint head, in multi pass printing side shooters and even failings canbe tolerated. Also the cost of a multi-pass printer is usually muchlower, especially for wide format substrates.

Curing Devices

The liquid UV curable inkjet ink according to the present invention iscured by ultraviolet radiation.

In inkjet printing, the UV curing device may be arranged in combinationwith the print head of the inkjet printer, travelling therewith so thatthe liquid UV curable inkjet ink is exposed to curing radiation veryshortly after been jetted.

In such an arrangement it can be difficult to provide a small enough UVradiation source connected to and travelling with the print head.Therefore, a static fixed radiation source may be employed, e.g. asource of curing UV-light, connected to the radiation source by means offlexible radiation conductive means such as a fibre optic bundle or aninternally reflective flexible tube.

Alternatively, the actinic radiation may be supplied from a fixed sourceto the radiation head by an arrangement of mirrors including a mirrorupon the radiation head.

The source of radiation arranged not to move with the print head, mayalso be an elongated radiation source extending transversely across theink-receiver surface to be cured and adjacent the transverse path of theprint head so that the subsequent rows of images formed by the printhead are passed, stepwise or continually, beneath that radiation source.

Any ultraviolet light source, as long as part of the emitted light canbe absorbed by the photo-initiator or photo-initiator system, may beemployed as a radiation source, such as, a high or low pressure mercurylamp, a cold cathode tube, a black light, an ultraviolet LED, anultraviolet laser, and a flash light. Of these, the preferred source isone exhibiting a relatively long wavelength UV-contribution having adominant wavelength of 300-400 nm. Specifically, a UV-A light source ispreferred due to the reduced light scattering therewith resulting inmore efficient interior curing.

UV radiation is generally classed as UV-A, UV-B, and UV-C as follows:

-   -   UV-A: 400 nm to 320 nm    -   UV-B: 320 nm to 290 nm    -   UV-C: 290 nm to 100 nm.

Furthermore, it is possible to cure the image using, consecutively orsimultaneously, two light sources of differing wavelength orilluminance. For example, the first UV-source can be selected to be richin UV-C, in particular in the range of 260 nm-200 nm. The secondUV-source can then be rich in UV-A, e.g. a gallium-doped lamp, or adifferent lamp high in both UV-A and UV-B. The use of two UV-sources hasbeen found to have advantages e.g. a fast curing speed and a high curingdegree.

In a particularly preferred embodiment, the UV curing is performed usingUV LEDs having an emission wavelength higher than 370 nm.

For facilitating curing, the inkjet printer often includes one or moreoxygen depletion units. The oxygen depletion units place a blanket ofnitrogen or other relatively inert gas (e.g. CO₂), with adjustableposition and adjustable inert gas concentration, in order to reduce theoxygen concentration in the curing environment. Residual oxygen levelsare usually maintained as low as 200 ppm, but are generally in the rangeof 200 ppm to 1200 ppm.

Substrates

There is no real limitation on the type of substrate for inkjet printingthe liquid UV curable inkjet ink of the invention on. The substrates mayhave ceramic, metallic, glass, wood, paper or polymeric surfaces forprinting. The substrate may also be primed, e.g. by a white ink.

The substrate may be porous, as e.g. textile, paper and card boardsubstrates, or substantially non-absorbing substrates such as e.g. aplastic substrate having a polyethylene terephthalate surface.

The substrate may also be pre-treated, e.g. by corona, plasma or flametreatment.

Preferred substrates including surfaces of polyethylene, polypropylene,polycarbonate, polyvinyl chloride, polyesters like polyethyleneterephthalate (PET), polyethylene naphthalate (PEN) and polylactide(PLA) and polyimide.

The substrate may also be a paper substrate, such as plain paper orresin coated paper, e.g. polyethylene or polypropylene coated paper.There is no real limitation on the type of paper and it includesnewsprint paper, magazine paper, office paper, wallpaper but also paperof higher grammage, usually referred to as boards, such as white linedchipboard, corrugated board and packaging board.

The substrates may be transparent, translucent or opaque. Preferredopaque substrates includes so-called synthetic paper, like the Synaps™grades from Agfa-Gevaert which are an opaque polyethylene terephthalatesheet having a density of 1.10 g/cm³ or more.

There is no restriction on the shape of the substrate. It can be a flatsheet, such a paper sheet or a polymeric film or it can be a threedimensional object like e.g. a plastic coffee cup. The three dimensionalobject can also be a container like a bottle or a jerry-can forincluding e.g. oil, shampoo, insecticides, pesticides, solvents, paintthinner or other type of liquids.

In a preferred embodiment of the inkjet printing method, the substrateis selected from textile, glass, pharmaceutical and food packaging.

In a preferred embodiment of the inkjet printing method, the substrateis a rigid medium selected from rigid PVC, paperboard, corrugated andwood.

In a preferred embodiment of the inkjet printing method, the substrateis substrate suitable for soft signage applications, such as banners,posters, POP/POS displays, indoor wall graphics, tradeshow displays,parasols, flags, outdoor advertising and backdrops.

A major advantage of the current liquid UV curable inkjet ink in textileinkjet printing is that not only a wide range of textiles can be printedupon, but also that the touch-and-feel is improved compared to standardUV curable inkjet inks. The reason is that the layer thickness obtainedafter printing the liquid UV curable inkjet inks of the presentinvention is much smaller than with the standard UV curable inkjet inks.

Suitable textiles can be made from many materials. These materials comefrom four main sources: animal (e.g. wool, silk), plant (e.g. cotton,flax, jute), mineral (e.g. asbestos, glass fibre), and synthetic (e.g.nylon, polyester, acrylic). Depending on the type of material, it can bewoven or non-woven textile.

The textile substrate is preferably selected from the group consistingof cotton textiles, silk textiles, flax textiles, jute textiles, hemptextiles, modal textiles, bamboo fibre textiles, pineapple fibretextiles, basalt fibre textiles, ramie textiles, polyester basedtextiles, acrylic based textiles, glass fibre textiles, aramid fibretextiles, polyurethane textiles (e.g. Spandex or Lycra™), Tyvek™ andmixtures thereof.

Suitable polyester textile includes polyethylene terephthalate textile,cation dyeable polyester textile, acetate textile, diacetate textile,triacetate textile, polylactic acid textile and the like.

Applications of these textiles include automotive textiles, canvas,banners, flags, interior decoration, clothing, hats, shoes, floor mats,doormats, brushes, mattresses, mattress covers, linings, sacking, stagecurtains, flame-retardant and protective fabrics, and the like.Polyester fibre is used in all types of clothing, either alone orblended with fibres such as cotton. Aramid fibre (e.g. Twaron) is usedfor flame-retardant clothing, cut-protection, and armor. Acrylic is afibre used to imitate wools.

The liquid UV curable inkjet inks of the invention are also verysuitable for inkjet printing on leather, especially on natural leather.

EXAMPLES Materials

All materials used in the examples were readily available from standardsources such as Sigma-Aldrich (Belgium) and Acros (Belgium) unlessotherwise specified.

PV19 is a C.I. Pigment Violet 19 pigment for which Ink Jet Magenta E5B02 from CLARIANT was used.

PR57 is a C.I. Pigment Red 57.1 pigment for which Symyler™ BrilliantCarmine 6B350SD from SUN CHEMICAL was used.

PY150 is a C.I. Pigment Yellow 150 pigment for which Cromophtal™ yellowD1085 from BASF was used.

5B550 is a carbon black pigment for which Special Black™ 550 from EVONIK(DEGUSSA) was used.

TIO2 is a titanium dioxide pigment available as Sachtleben RDI-S fromSACHTLEBEN.

PB15:4 is an abbreviation used for Hostaperm™ Blue P-BFS, a C.I. PigmentBlue 15:4 pigment from CLARIANT.

E7701 is a polyacrylate dispersion agent available as Efka™ 7701 fromBASF.

DB162 is an abbreviation used for the polymeric dispersant Disperbyk™162 available from BYK CHEMIE GMBH whereof the solvent mixture of2-methoxy-1-methylethylacetate, xylene and n-butylacetate was removed.The polymeric dispersant is a polyester-polyurethane dispersant on thebasis of caprolacton and toluene diisocyanate having an amine value of13 mg KOH/g, a Mn of about 4,425 and a Mw of about 6,270.

535000 is an abbreviation used for SOLSPERSE™ 35000, apolyethyleneimine-polyester hyperdispersant from LUBRIZOL.

DPGDA is dipropylene glycol diacrylate available as Sartomer™ SR508 fromARKEMA.

PEA is 2-phenoxyethyl acrylate available as Sartomer™ SR339C fromARKEMA.

VEEA is 2-(2′-vinyloxyethoxy)ethyl acrylate, a difunctional monomeravailable from NIPPON SHOKUBAI, Japan.

MPDA is 3-methyl 1,5-pentanediol diacrylate available as Sartomer™ SR341from ARKEMA.

(EO)3TMPTA is an ethoxylated (EO3) trimethylolpropanetriacrylateavailable as Miramer™ M3130 from RAHN.

CN3755 is an acrylated amine synergist available as Sartomer™ CN 3755from ARKEMA.

C704 is an acrylated polyester adhesion promoter available as Sartomer™CN704 from ARKEMA.

EBLEO is a polyfunctional acrylated photoinitiator available as EBECRYL™LEO 10101 from ALLNEX.

STABI-1 and STABI-2 are mixtures forming a polymerization inhibitorhaving a composition according to Table 1, wherein DPGDA or PEA is usedin order to arrive at the desired wt % of polyfunctional andmonofunctional polymerizable compounds.

TABLE 1 wt % of Component STABI-1 STABI-2 PEA 82.4 — DPGDA — 82.4p-methoxyphenol — 4.0 2,6-di-tert-buty1-4- methylphenol — 10.0Cupferron ™ AL — 3.6

Cupferron™ AL is aluminum N-nitrosophenylhydroxylamine from WAKOCHEMICALS LTD.

STABI-3 is 4-hydroxy-2,2,6,6-tetramethylpiperidinooxy sebacate availableas Irgastab™ UV 10 from BASF.

TPO-L is 2,4,6-trimethylbenzoyl phenylphosphinic acid ethyl esteravailable as Lucirin™ TPO-L from BASF.

BAPO is a bis(2,4,6-trimethylbenzoyl)-phenylphosphineoxidephotoinitiator available as Irgacure™ 819 from BASF.

DETX is 2,4-diethylthioxanthone available as Genocure™ DETX from RAHN.

GAB is a polymeric tertiary amine available as Genopol™ AB-1 from RAHN.

Tegorad™ 2100 is a radically cross-linkable silicone acrylate availablefrom EVONIK. US 2011086221 (3M) describes Tegorad™ 2100 as:

wherein n ranges from 10 to 20 and m ranges from 0.5 to 5.

C7500 is a silicone surfactant available as COATOSIL™ 7500 fromMOMENTIVE PERFORMANCE MATERIALS.

UV3510 is Byk™ UV3510, a polyether modified polydimethylsiloxane,supplied by BYK Chemie GmbH.

AXANTH is a polymerizable thioxanthone according to Formula (AX-1):

This photoinitiators was synthesized as follows:

Step 1: The Aminolysis of Omnipol™ TX

395 g Omnipol™ TX, supplied by IGM, was dissolved in 1850 ml dimethylsulfoxide. The reaction mixture was heated to 60° C. and 363 g (3 mol)tris(hydroxymethyl)aminomethane and 415 g (3 mol) potassium carbonatewere added. The reaction was allowed to continue for 2 hours at 60° C.The reaction mixture was allowed to cool down to room temperature. Theprecipitated salts were removed by filtration and the reaction mixturewas added to a mixture of 1500 ml water and 250 ml acetone. Theintermediate thioxanthone precipitated from the medium, was isolated byfiltration and dried. The crude thioxanthone was treated with 1500 mlacetone, isolated by filtration and dried. 260 g of the thioxanthone wasisolated (TLC-analysis: RP-C18 (Partisil™ KC18F, supplied by Whatman),eluent MeOH/0.5 M NaCl, R_(f)=0.55). TLC analysis showed the presence ofa small amount of an isomeric structure (R_(f)=0.60). The followingstructure was assigned to the isomer:

The intermediate was further used as a mixture of the main isomer andthe minor isomer.

Step 2: The Addition to VEEA:

22 g (58 mmol) of the amido-trihydroxy-thioxanthone was added to 227.8 g(1.224 mol) VEEA. 0.13 g (86 μl, 1.16 mmol) trifluoroacetic acid and0.25 g (1.16 mmol) BHT were added and the mixture was heated to 77° C.The reaction was allowed to continue at 77° C. for 16 hours. Thereaction was allowed to cool down to room temperature and 20 g ofactivated Lewatit M600 MB was added. The mixture was stirred for fourhours at room temperature. The ion exchanger was removed by filtration.AX-1 was used as a solution in VEEA. (TLC-analysis: RP-C18 (Partisil™KC18F, supplied by Whatman), eluent: MeOH/0.5 M NaCl 80/20, R_(f)=0.18).Based on ¹H-NMR analysis, the solution contained 19 wt % AX-1.

PET100 is a 100 μm unsubbed PET substrate with on the backside ananti-blocking layer with antistatic properties available fromAGFA-GEVAERT as P100C PLAIN/ABAS.

Measurement Methods 1. Viscosity

The viscosity of the UV curable inkjet inks was measured at 25° C. or45° C. and at a shear rate of 10 s⁻¹ or 1,000 s⁻¹ using a Rotovisco™ RV1viscometer from HAAKE.

2. Flexibility

A liquid UV curable inkjet ink was coated on a Metamark™ MD5-100substrate using a bar coater. The coated sample was fully cured using aFusion DRSE-120 conveyer, equipped with a Fusion VPS/I600 lamp (D-bulb),which transported the samples under the UV-lamp on a conveyer belt at aspeed of 20 m/min.

The flexibility was determined using a custom built apparatus shown inFIG. 5 for stretching a strip having a length of 8 cm and a width of 1cm obtained from the coated sample using a cutter. The strip was mountedbetween a first fixed wall and a second wall which could be horizontallydisplaced by rotation of a handle.

The strip was elongated from an original length L1 of 5 cm to the lengthL2 at which the strip ruptured. The elongation was calculated as apercentage according to Formula (III):

Elongation(%)=(L2−L1/L1)×100   Formula (III).

If no rupture of the strip was observed at an elongation of 150%, thenthe flexibility was evaluated as “>150%”. Acceptable flexibility meansan elongation of at least 40%.

3. Curing Speed

A liquid UV curable inkjet ink was coated on a PET100 substrate using abar coater and a 10 μm wired bar. The coated sample was cured using aFusion DRSE-120 conveyer, equipped with a Fusion VPS/I600 lamp (D-bulb),which transported the samples under the UV-lamp on a conveyer belt at aspeed of 20 m/min. The maximum output of the lamp was 1.05 J/cm² and apeak intensity of 5.6 W/cm². The percentage of the maximum output of thelamp was taken as a measure for curing speed, the lower the number thehigher the curing speed. A sample was considered as fully cured at themoment scratching with a Q-tip caused no visual damage. Table 2 showsthe maximum peak intensity (MPI) in W/cm² and the dose in J/cm² of theD-bulb for the different UV regions measured with a UV Power Puck 8651from EIT Inc. (USA) at different settings of the lamp output for a beltspeed of 20 m/min.

TABLE 2 UVC UVB UVA UVF (250-260 (280-320 (320-390 (395-445 Lamp nm) nm)nm) nm) Output MPI Dose MPI Dose MPI Dose MPI Dose 100% 0.06 0.01 0.780.14 3.16 0.59 1.63 0.31 80% 0.05 0.01 0.55 0.11 2.12 0.40 1.10 0.20 60%0.04 0.01 0.42 0.08 1.35 0.26 0.64 0.12 40% 0.03 0.01 0.26 0.05 0.510.09 0.24 0.04

The curing speed should be less than 100%, preferably no more than 95%as lamp power decreases with aging. A curing speed higher than 100%means that the coated sample had to be conveyed under the lamp more thanonce.

4. Scratch Resistance

A liquid UV curable inkjet ink was coated on a PET100 substrate using abar coater. The coated sample was fully cured using a Fusion DRSE-120conveyer, equipped with a Fusion VPS/I600 lamp (D-bulb), whichtransported the samples under the UV-lamp on a conveyer belt at a speedof 20 m/min.

The scratch resistance of the coated sample of a liquid UV curableinkjet was determined according to ISO 4586-2:2004 (E)/ASTM C1624 usinga Rockwell indenter with parameters: Speed 30 mm/sec; Load: 10-200 mN;Test area length 100 mm; and Tip: diamond: r 15 μm, 90°.

This test ran a diamond topped needle across the ink surface while thepressure onto the needle was increased. The test started with a pressureof 10 mN and then went up to 200 mN. The result of the Rockwell indenterscratch test was always the pressure value which revealed the firstmicroscopic cracks in the pressure trail of the needle. So, the positionwhere the needle started to penetrate the ink layer, marked the maximumforce applicable to the layer without scratching it. Using a microscope,the evaluation of the scratch resistance was made in accordance with theclassification described in Table 3.

TABLE 3 Classification Evaluation +++ No scratch observed at 120 mN ++Scratch observed between 70 mN to less than 120 mN + Scratch observedbetween 60 mN to less than 70 mN − Scratch observed between 50 mN toless than 60 mN −− Scratch observed at less than 50 mN

5. Optical Density

The optical density was measured in reflection using a Macbeth™ TR924spectrodensitometer using a visual filter.

5. Average Particle Size

The particle size of pigment particles in a pigment dispersion wasdetermined by photon correlation spectroscopy at a wavelength of 633 nmwith a 4 mW HeNe laser on a diluted sample of the pigment dispersion.The particle size analyzer used was a Malvern™ nano-S available fromGoffin-Meyvis.

The sample was prepared by addition of one drop of pigment dispersion toa cuvette containing 1.5 mL ethyl acetate and mixed until a homogenoussample was obtained. The measured particle size is the average value of3 consecutive measurements consisting of 6 runs of 20 seconds.

6. Dynamic Viscosity

The dynamic viscosity was determined using a DHR2-rheometer from TAInstruments equipped with a 60 mm 1° stainless steel cone. For measuringat a specific temperature, a conditioning step was performed withoutpre-shear. Upon measuring the shear rate was increased logarithmicallybetween 1 en 1000 s⁻¹ with 5 points per decade.

Example 1

This example illustrates the improvements in scratch resistance andflexibility when using UV curable inkjet inks according to the presentinvention.

Preparation of Pigment Dispersion CYAN1

The pigment dispersion CYAN1 was made by mixing the components accordingto Table 4 for 30 minutes using a DISPERLUX™ disperser from DISPERLUXS.A.R.L., Luxembourg. The dispersions were then milled using a BachofenDYNOMILL ECM mill filled with 0.4 mm yttrium stabilized zirconia beads(“high wear resistant zirconia grinding media” from TOSOH Co.). Themixtures were circulated over the mill for 2 hours. After milling, thepigment dispersions were discharged over a 1 μm filter into a vessel.

TABLE 4 Component wt % PB15:4 20.0 E7701 13.3 STABI-1 1.0 PEA 65.7

The cyan pigment dispersion CYAN1 had an average particle size of 124nm.

Preparation of Pigment Dispersion CYAN2

The pigment dispersion CYAN2 was made in the same way as the dispersionCYAN1, except that the components according to Table 5 were used.

TABLE 5 Component wt % PB15:4 15.0 E7701 15.0 STABI-2 1.0 DPGDA 69.0

The cyan pigment dispersion CYAN2 had an average particle size of 119nm.

Preparation of Liquid UV Curable Inkjet Inks

The liquid UV curable inkjet inks C-1 to C-8 and I-1 to 1-4 wereprepared by mixing the ink components according to Table 6 and Table 7.The weight percentages (wt %) are based on the total weight of theliquid UV curable inkjet ink. The viscosity of each inkjet ink wasmeasured for the conditions shown in the tables.

TABLE 6 wt % of: C-1 C-2 C-3 C-4 C-5 C-6 C-7 C-8 DPGDA — 19.50 39.3559.20 70.00 — 35.40 0.75 PEA 65.10 45.60 25.75 5.90 — 18.80 — — STABI-10.80 0.80 0.80 0.80 — 0.40 — — STABI-2 — — — — 0.90 0.50 0.15 GAB 5.005.00 5.00 5.00 5.00 5.00 5.00 5.00 TPO-L 6.00 6.00 6.00 6.00 6.00 6.006.00 6.00 BAPO 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 C7500 0.10 0.100.10 0.10 0.10 0.10 0.10 0.10 CYAN1 20.00 20.00 20.00 20.00 — 66.70 — —CYAN2 — — — — 15.00 — 50.00 85.00 Polymerizable composition wt % on ink79.1 79.1 79.1 79.1 80.0 65.5 68.7 58.0 % DPGDA — 25.0 50.0 75.0 100.0 —100.0 100.0 % PEA 100.0 75.0 50.0 25.0 — 100.0 — — Viscosity (mPa · s)25° C. 24.1 23.5 23.5 22.6 18.0 96.3 49.5 — 1,000 s⁻¹ 45° C. 11.0 10.710.7 10.9 9.3 39.2 23.5 — 1,000 s⁻¹ 25° C. — — — — — 98.0 90.3 200.1 10s⁻¹ 45° C. — — — — — 40.5 27.5 88.2 10 s⁻¹

TABLE 7 wt % of: I-1 I-2 I-3 I-4 DPGDA 17.10 1.00 18.20 4.55 PEA 1.7034.40 17.20 16.00 STABI-1 0.40 — — — STABI-2 — 0.50 0.50 0.35 GAB 5.005.00 5.00 5.00 TPO-L 6.00 6.00 6.00 6.00 BAPO 3.00 3.00 3.00 3.00 C75000.10 0.10 0.10 0.10 CYAN1 66.70 — — — CYAN2 — 50.00 50.00 65.00Polymerizable composition wt % on ink 65.5 68.7 68.7 64.0 % DPGDA 25.050.0 75.0 75.0 % PEA 75.0 50.0 25.0 25.0 Viscosity (mPa · s) 25° C. 91.558.2 82.6 — 1,000 s⁻¹ 45° C. 30.8 24.1 24.4 — 1,000 s⁻¹ 25° C. 66.2 51.043.7 127.2   10 s⁻¹ 45° C. 24.9 24.8 24.4 43.7   10 s⁻¹

Evaluation and Results

The scratch resistance, flexibility and curing speed were determinedaccording to the above described test methods using coatings made by abar coater. When printing inkjet inks having a higher pigmentconcentration, the ink lay-down can be reduced for obtaining the sameoptical density. So for a fair comparison, ink layers from a moreconcentrated inkjet ink should be coated at a thinner layer thicknessthan a less concentrated inkjet ink. The thickness of a layer coated bya bar coater at room temperature is mainly influenced by viscosity andthe type of bar used in the bar coater. The best results for faircomparison were obtained according to Table 8.

TABLE 8 Ink Bar Optical coating coater Density C-1 20 μm 2.2 wired barC-2 20 μm 2.3 wired bar C-3 20 μm 2.4 wired bar C-4 20 μm 2.4 wired barC-5 20 μm 2.4 wired bar C-6  4 μm 2.1 wired bar C-7  4 μm 2.1 wired barC-8 polished bar 2.1 I-1  4 μm 2.3 wired bar I-2  4 μm 2.3 wired bar I-3 4 μm 2.3 wired bar I-4 polished bar 1.7

The results for scratch resistance, flexibility and curing speed areshown by Table 9.

TABLE 9 Polyfunctional wt % polymerizable Scratch Curing Ink Pigmentcompound resistance Flexibility speed C-1 3.0  0.0 wt % − − >150 60 C-23.0  25.0 wt % − 68 50 C-3 3.0  50.0 wt % ++ 37 55 C-4 3.0  75.0 wt % ++25 55 C-5 3.0 100.0 wt % ++ 12 65 C-6 10.0  0.0 wt % − − >150 70 C-710.0 100.0 wt % +++ 34 80 C-8 17.0 100.0 wt % ++ >150 100 I-1 10.0  25.0wt % ++ 136 80 I-2 10.0  50.0 wt % ++ 60 80 I-3 10.0  75.0 wt % ++ 54 75I-4 13.0  75.0 wt % ++ >150 95

From Table 9, it should be clear that surprisingly an improvedcompromise on scratch resistance and flexibility is found with theinventive inkjet inks I-1 to I-4. An inkjet ink containing onlymonofunctional or otherwise only polyfunctional polymerizable compoundsfails on scratch resistance respectively flexibility.

A disadvantage is that the curing speed apparently decreases somewhat,so the pigment concentration is preferably limited to 13.0 wt % tominimize this effect. It is not clear why the curing speed decreaseseven though in comparison a slightly higher amount of photoinitiators ispresent for a smaller polymerizable composition in the inkjet ink. Onehypothesis is that the thinner ink layers are more susceptible to oxygeninhibition of the free radical polymerization upon UV curing.

Example 2

This example illustrates the effect of pigment concentration and jettingviscosity on reliability of the inkjet printing process, moreparticularly on the tail length and number of satellites of ejected inkdroplets.

Preparation of Liquid UV Curable Inkjet Inks

The liquid UV curable inkjet inks Ink-1 to Ink-5 were prepared by mixingthe components according to Table 10 using a cyan pigment dispersion ofPB15:4 in VEEA or DPGDA with DB162 or S35000 as polymeric dispersant.The pigment dispersions were made in the same manner as described abovefor the pigment dispersion CYAN1. By selecting different type and amountof monomers and polymeric dispersant and monomers, different viscositiesabove 16 mPa·s at 45° C. were realized for the inkjet inks.

TABLE 10 wt % of component Ink-1 Ink-2 Ink-3 Ink-4 Ink-5 PB15:4 2.502.50 5.00 8.50 8.60 DB162 2.50 2.50 — — 8.60 S35000 — — 2.50 8.50 — VEEA11.48 11.48 — — 43.45 DPGDA — 13.55 23.46 35.61 24.00 MPDA 10.00 — 9.509.50 — (EO) 3TMPTA 42.42 9.50 34.35 12.48 — Tegorad ™ 2100 — 15.00 — — —CN3755 — — 9.52 9.52 — TPO-L — — 4.76 4.76 6.00 EBLEO10101 30.00 25.00 —— — GAB — — 5.56 5.56 5.00 AXANTH — 20.00 — — — BAPO — — 2.38 2.38 3.00STABI-1 1.00 0.17 0.96 1.18 1.25 STABI-3 — 0.20 — — — C7500 0.10 0.102.00 2.00 0.10

Evaluation and Results

Different jetting temperatures were used so that at least two differentviscosities were obtained with the same inkjet ink. The viscosity wasmeasured at a shear rate of 10 s⁻¹ and a temperature indicated by Table11.

The jetting was performed using a through-flow piezoelectric DOD printhead CF1ou from TOSHIBA TEC. The jetting voltage was adapted so that thesatellite analysis was performed at the same drop velocity of 6 m/s forall inkjet inks.

The tail length and number of satellites were determined using adrop-in-flight analysis tool JetXpert™ from ImageXpert for a largenumber of ejected ink droplets. An average was calculated and rounded tothe nearest integer. The results are shown in Table 11 and FIG. 4.

TABLE 11 Pigment Jetting Tail Inkjet Con- Temp. Viscosity Voltage lengthNumber of ink centration (° C.) (mPa · s) (V) (μm) satellites Ink-1 2.549 26 22 461 5 Ink-1 2.5 42 34 25 516 6 Ink-2 2.5 49 34 23 568 5 Ink-22.5 46 37 25 593 7 Ink-2 2.5 41 45 29 691 8 Ink-3 5.0 47 37 22 467 4Ink-3 5.0 38 51 27 510 4 Ink-4 8.5 51 55 27 530 3 Ink-4 8.5 47 62 29 5482 Ink-5 8.6 49 28 20 392 4 Ink-5 8.6 31 51 29 508 3

From Table 11, it can been seen that inkjet inks Ink-1 and Ink-2containing a state-of-the-art amount (2.5 wt %) of organic colourpigment exhibit also at a higher jetting viscosity above 16.0 mPa·s alarge number of satellites. By comparing the UV curable inkjet inksInk-1, Ink-2 and Ink-5 having a comparable jetting viscosity at 49° C.,it should be clear that the increased pigment concentration results infewer satellites and thus better image quality. The results have beenvisualized in FIG. 4. At a pigment concentration of 5 wt % in the inkjetink, no change in the number of satellites is observed. It is submittedthat from a concentration of 6.0 wt % of organic colour pigment in theinkjet ink, a clear reduction in number of satellites can be observedwith increasing jetting viscosity. The preferred range for jettingviscosity is between 45° C. and 65° C. Below 45° C. the number ofsatellites remains somewhat higher, while above 65° C. the reliabilityof the inkjet printing process is reduced as undesired free radicalpolymerization of the inkjet ink can sometimes be observed at highertemperatures.

Example 3

This example illustrates a UV curable CMYKW inkjet ink set.

Preparation of Pigment Dispersions Pigment Dispersion for Ink Y

The pigment dispersion was made by mixing the components according toTable 12 for 30 minutes using a DISPERLUX™ disperser from DISPERLUXS.A.R.L., Luxembourg. The dispersions were then milled using a BachofenDYNOMILL ECM mill filled with 0.4 mm yttrium stabilized zirconia beads(“high wear resistant zirconia grinding media” from TOSOH Co.). Themixtures were circulated over the mill for 2 hours. After milling, thepigment dispersions were discharged over a 1 μm filter into a vessel.

TABLE 12 Component wt % PY150 18.0 E7701 12.0 STABI-1 1.0 DPGDA 69.0

Pigment Dispersion for Ink M

The pigment dispersion was prepared in the same manner as for Ink Y,except that the components according to Table 13 were used.

TABLE 13 Component wt % PV19 15.0 E7701 10.0 STABI-1 1.0 VEEA 25.0 DPGDA49.0

Pigment Dispersion for Ink C

The pigment dispersion was prepared in the same manner as for Ink Y,except that the components according to Table 14 were used.

TABLE 14 Component wt % PB15:4 15.0 E7701 10.0 STABI-1 1.0 VEEA 37.5DPGDA 36.5

Pigment Dispersion for Ink K

The pigment dispersion was prepared in the same manner as for Ink Y,except that the components according to Table 15 were used.

TABLE 15 Component wt % SB550 9.1 PB15:4 4.2 PR57 1.7 E7701 7.5 STABI-11.0 VEEA 29.0 DPGDA 47.5

Pigment Dispersion for Ink W

The pigment dispersion was prepared in the same manner as for Ink Y,except that the components according to Table 16 were used.

TABLE 16 Component wt % TIO2 50.0 DB162 4.0 STABI-2 1.0 VEEA 46.0

Preparation of Liquid UV Curable Inkjet Inks

The UV curable CMYKW inkjet ink set was prepared by mixing thecomponents for each liquid UV curable inkjet ink according to Table 17using the above prepared pigment dispersions.

TABLE 17 wt % of component Ink Y Ink M Ink C Ink K Ink W PY150 8.55 — —— — PB15:4 — — 8.57 2.08 — PV19 — 8.57 — — — PR57 — — — 0.86 — SB550 — —— 4.56 — TIO2 — — — — 26.10 E7701 5.70 5.71 5.71 3.75 — D162 — — — —2.10 DPGDA 33.78 29.00 20.86 34.92 24.00 VEEA 27.75 32.50 36.64 23.7523.60 CN3755 9.52 9.52 9.52 9.52 — C704 — — — — 9.00 TPO-L 4.76 4.764.76 6.00 6.00 GAB 5.56 5.56 5.56 5.56 5.00 BAPO 2.38 2.38 2.38 3.003.00 DETX — — 4.00 4.00 — UV3510 1.00 1.00 1.00 1.00 — C7500 — — — —0.10 STABI-1 1.00 1.00 1.00 1.00 1.10

Evaluation and Results

The dynamic viscosity was determined for each liquid UV curable inkjetink at 25, 35 and 45° C. The results are shown by Table 18.

TABLE 18 Dynamic Viscosity @shear rate 1000 s⁻¹ (mPa · s) TemperatureInk Y Ink M Ink C Ink K Ink W 25° C. 52.9 54.0 49.3 44.5 45.8 35° C.35.5 35.6 33.5 29.5 29.9 45° C. 25.6 26.0 23.7 20.6 20.7

The average particle size was determined after 1 week at 60° C. From theresults in Table 19, it can be seen that the particle size of theorganic pigments remains less than 150 nm. The particle size of thewhite inkjet ink is higher than 200 nm, thereby guaranteeing goodopacity.

TABLE 19 Inkjet Average Ink Particle Size Ink Y 144 nm Ink M 140 nm InkC 116 nm Ink K 140 nm Ink W 235 nm

Multi-colour images of good quality were printed with the UV curableCMYKW inkjet ink set on a plasma-treated polypropylene substrate using acustom built inkjet printer employing through-flow piezoelectric DODprint heads CF1ou from TOSHIBA TEC at a drop velocity of 6 m/s.

REFERENCE SIGNS LIST

TABLE 20 1 Print head 2 Ejected droplet 3 Ink-receiver 4 Tail 5 Maindroplet 6 Fast satellite 7 Slow satellite 8 Ink dot 9 Secondary ink dot

1-15. (canceled)
 16. A liquid UV curable inkjet ink comprising: one or more photoinitiators; an organic color pigment; and a polymerizable composition including at least one monofunctional polymerizable compound and at least one polyfunctional polymerizable compound; wherein the organic color pigment is present in an amount of 6.0 to 13.0 wt % based on a total weight of the liquid UV curable inkjet ink; the at least one polyfunctional polymerizable compound is present in an amount of at least 20.0 wt % based on a total weight of the polymerizable composition; and the liquid UV curable inkjet ink has a viscosity at 45° C. and a shear rate of 10 s⁻¹ of at least 16.0 mPa·s.
 17. The liquid UV curable inkjet ink according to claim 16, wherein the at least one polyfunctional polymerizable compound is present in an amount of 25.0 to 50.0 wt % based on the total weight of the polymerizable composition.
 18. The liquid UV curable inkjet ink according to claim 16, wherein at least one of the one or more photoinitiators is selected from the group consisting of a polymeric photoinitiator, a polymerizable photoinitiator, and a photoinitiator including a plurality of photoinitiating groups.
 19. The liquid UV curable inkjet ink according to claim 16, wherein the polymerizable composition is present in amount less than 70.0 wt % based on the total weight of the liquid UV curable inkjet ink.
 20. The liquid UV curable inkjet ink according to claim 16, further comprising 0 to 10 wt % of an organic solvent based on the total weight of the liquid UV curable inkjet ink.
 21. The liquid UV curable inkjet ink according to claim 16, wherein the viscosity is from 20.0 to 65.0 mPa·s at 45° C. and a shear rate of 10 s⁻¹.
 22. The liquid UV curable inkjet ink according to claim 16, wherein the one or more photoinitiators includes an acylphosphine oxide photoinitiator and a thioxanthone photoinitiator.
 23. A UV curable inkjet ink set for printing different colors comprising: at least one liquid UV curable inkjet ink according to claim
 16. 24. The UV curable inkjet ink set according to claim 23, wherein the at least one liquid UV curable inkjet ink includes at least three liquid UV curable color inkjet inks, each including one or more different organic color pigments present in an amount of more than 5.0 wt % based on the total weight of the liquid UV curable color inkjet ink.
 25. The UV curable inkjet ink set according to claim 23, further comprising: a liquid UV curable white inkjet ink including a titanium dioxide pigment having an average particle size larger than 180 nm.
 26. The UV curable inkjet ink set according to claim 23, further comprising: a liquid UV curable black ink including a carbon black pigment and a β-copper phthalocyanine pigment having an average particle size smaller than 200 nm.
 27. A UV curable inkjet printing method comprising the steps of: jetting at least one liquid UV curable inkjet ink according to claim 16 onto a substrate with a print head at a jetting temperature of 45° C. or more; and UV curing the at least one liquid UV curable inkjet ink jetted on the substrate.
 28. The UV curable inkjet printing method according to claim 27, wherein the substrate is pre-treated by a corona, a plasma, or a flame treatment.
 29. The UV curable inkjet printing method according to claim 27, wherein the print head is a through-flow piezoelectric drop-on-demand print head.
 30. The UV curable inkjet printing method according to claim 27, wherein the at least one liquid UV curable inkjet ink is jetted at a temperature between 45° C. and 65° C. 